1. Field of the Invention
The present invention relates to a direct color thermal printing method using a thermosensitive color recording material which is colored when heated. The present invention also relates to a direct color thermal printer.
2. Related Art
A thermosensitive color recording material has been suggested, for example, in Japanese Laid-open Patent Application 61-213169, having thermosensitive coloring layers for yellow, magenta and cyan which are laminated or formed on a supporting material in this order from the outside. In this type of recording material, the heat sensitivities of the thermosensitive coloring layers (hereinafter referred to as coloring layers) become lower as the distance from the outside surface increases. Furthermore, the coloring layers have properties that each coloring layer is optically fixed by electromagnetic rays of a respective specific wave length range.
When recording a full-color image on the above-described thermosensitive color recording material, a thermal head having a plurality of heating elements arranged in a line is used. First, the coloring layer for yellow, or the first layer that is disposed on the outermost position of the coloring layers, is thermally recorded, while the thermal head is moved relative to the thermosensitive color recording material. After recording a yellow frame of the full-color image in the first layer in this way, the thermosensitive color recording material is exposed to light having a wave length range by which a diazonium salt compound contained in the first layer is discomposed. Thereby, the first layer is optically fixed by discomposing a part of the diazonium salt compound that still has a capacity for coupling.
Next, a magenta frame of the full-color image is recorded in the coloring layer for magenta, or the second layer that is disposed in the second place from the outside, by using a higher heat energy than that applied for the yellow frame recording. Thereafter, the second layer is optically fixed by being exposed to light having a wave length range that discomposes a diazonium salt compound in the second layer. Then, the highest heat energy is applied to the thermosensitive color recording material, so as to record a cyan frame of a full-color image in the coloring layer for cyan, that is, the third layer disposed at the innermost position of the coloring layers. Finally, light having a wave length range that discomposes a diazonium salt compound, is applied to optically fix the third layer.
It is also known to apply a bias pulse having a relatively large width for heating the thermosensitive color recording material up to a predetermined temperature and then to apply a number of gradation pulses having less than the relatively large width for heating the recording material to the heating element, for recording a pixel in the above-described thermosensitive color recording material, wherein the number of the gradation pulses is determined in accordance with the gradation level of the pixel.
The pulse durations of the bias pulse and the gradation pulse are set larger, when the heat sensitivity of the color layer becomes lower, in order to set a long heating time for the low heat sensitive coloring layer. Therefore, the conventional direct color thermal printing method uses a constant thermal recording speed or a constant recording cycle of one pixel that is adapted to the third layer having the lowest heat sensitivity, for recording in the three coloring layers. As a result, the conventional method has a problem because the total printing time necessary for recording a full-color image is long.
Moreover, because of such a constant recording speed, the cooling time for cooling the heating element within a constant recording cycle of one pixel, changes according not only to the gradation level of each pixel but also according to the heat sensitivity of the coloring layer. Therefore, the cooling time for recording the first layer tends to be longer than the cooling time period for recording the third layer, which is practically redundant. This may make the heating element too cool, so that a lowered efficiency of the heat energy results.
If on the other hand, the heating elements are energized for a longer time by using a lower level electric power, in order to prevent the cooling time period from being too long, the heat energy is transmitted to the second layer while recording the first layer. Thereby, the second layer may be finely colored unnecessarily. The coloring of the unnecessary portion of the second layer causes the reproduction to have an improper tone.